LCDs and CRTs are widely employed in a variety of typical display devices such as television sets, computer terminals and the like. A key problem is improving the quality of the display devices in keeping with the trend for increased image resolution. With the advent of multimedia including, in particular, a variety of portable terminals of communication systems represented by mobile telephones and the like, innovative display systems are expected to play a very important role in the interface between man and machine. Among the concerns is improved abrasion resistance for such displays. U.S. Pat. No. 5,914,073 suggests an adhesively applied laminate of a clear resin overcoated with a polyol acrylate resin that is UV cured. U.S. Pat. No. 6,329,041 suggests a three layer hard coating with a middle buffer layer overcoated with an antireflection coating. U.S. Pat. No. 6,376,060 suggest including an inorganic filler in a hardcoat film.
LCDs play a big role in this market of portable terminals since they are light in weight and can be made compact along with their versatility for many types of displays. Since these portable terminals are frequently used outdoors, it is important to ensure good visibility of their images even in bright sunlight by suppressing glare or specular reflection as completely as possible. In order to ensure this, an antiglare film is preferably provided on the surface of the display for diffusing external light and suppressing specular reflection.
U.S. Pat. No. 5,998,013 discloses an antiglare film formed by coating a resin, containing fillers such as agglomerated silicon dioxide, onto the surface of a transparent substrate film. Another method of achieving the same is by texturing or roughening the surface of the substrate. For example, the surface of a substrate can be directly roughened by sandblasting, or embossing or the like, or by employing a method in which a porous film is formed on the surface of the substrate.
U.S. Pat. No. 6,008,940, describes a glare-proof layer comprising a resin with coarse and fine particles and a refractive index of 1.4-1.6. The fine particles are hydrophilic and have moisture contents of greater than 0.5 percent. U.S. Pat. No. 6,217,176 describes an antiglare film comprising a resin containing two types of light-transparent fine particles to control the index of refraction of the layer. U.S. Pat. No. 6,074,741 describes an antiglare material comprising a roughened surface layer derived from an ultraviolet curable resin containing an epoxy compound and a photo cationic polymerization initiator and two different populations of resin beads. U.S. Pat. No. 6,347,871 describes an antiglare layer comprising two resin coated layers wherein the, top layer contains particles smaller in size than those in the bottom layer. U.S. Pat. No. 6,343,865 relates to a non glare film onto which a low refractive index film is laminated resulting in suppressed degradation of contrast and whitening.
More recently, displays such as those with touch screens have gained prominence. Hence there has been an increasing demand for development of display screens that have good abrasion resistance and resistance to handling defects in general while providing antiglare properties.
While the prior art describes the use of resin beads and particles in antiglare coatings there is no teaching directed towards the morphology of the particles. Whereas spherical symmetric polymer particles can provide the desired antiglare properties, their highly curved surfaces can potentially disadvantage the system. When abraded, such spherical particles have a tendency to come off from the coating due to poor adhesion at the particle/binder interface. This leads to dusting and microscopic pitting of the surface resulting in increased transmission haze and reduced image contrast and sharpness. Additionally, when antiglare coatings and hard coats are coated on flexible substrates such as cellulose triacetate, adhesion to the substrate becomes increasingly important in light of the fact that such flexible substrates are often handled in wound roll form.
It is well known in the industry to use aggregated silica particles in coatings to provide antiglare properties. While these particles provide controlled haze and gloss, they can contribute to cinch abrasion in such wound rolls of flexible substrates.
Further, it is well known in the industry to use radiation curable monomers and oligomers for an abrasion resistant coating. Most of these are coated from organic solvents. While the prior art goes on to list various solvents that provide good adhesion of the coating to the base material there is no mention of the adverse effects produced by some of the listed solvents in the abrasion resistant properties of the final coating. When solvents are used that can penetrate the base support material and release additives present therein such as plasticizers, into the coating, the modulus of the coating is compromised resulting in less than excellent abrasion resistance and pencil hardness. Therefore, selection of solvent from which to apply the hard coat onto the substrate of choice is very critical.
In spite of the teachings in the arena of antiglare abrasion resistant screens, there exists a continuing need for developing a robust display screen for the reasons discussed above with either antiglare characteristics which prevent specular reflection of external light from sources such as the sun, fluorescent lamps and the like on them or excellent abrasion resistance or both since the antiglare film is coated on the outermost surface of the polarizer in the LCD.